A comparative study for two different series,La0.67Ca0.33Mn1-xCrxO3 and La0.67+xCa0.33-xMn1-x CrxO3,is performed with x changing from 0.04 to 0.08 through measurements of zero-field resistivity (ρ) magnetoresistance(MR) and ac susceptibility (x) as functions of temperature(T). For the second group,a single insulator-metal transition was characterized by the resistivity maximum appears in ρ VS.T curve.For the first group,besides the resistivity peak appearing at higher temperatures,the sample shows evidence for the existence of another peak appearing at lower temperatures.Compared to the first group,the second group shows a significant enhancement in MR effect,while the measurement of x-T dependence does not display obvious differences for the two series.Different behaviors observed in the tWO series are discussed by considering possible exchange interaction between Mn3+ ion and Cr3+ ion.
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